In addition to smaller and thinner size, various kinds of functions are required to an optical information recording and reproduction apparatus such as an optical disk drive or unit.
In a conventional normal optical disk unit using one light source, a high-speed data transfer is desired since information validation after the information has been recorded is performed by rotating a disk again, which leads to a slow data transfer speed. In Japanese Patent Laid-Open 64-70936 gazette, a multi-beam type optical head is suggested in which two semiconductor laser chips are arranged in an optical head in a opposing manner, two parallel beams are formed by the same collimating lens, the two beams are made to be incident to a focusing lens, two optical spots are radiated on the same track on an optical disk surface, and the information recording and reproduction for the information validation are performed at a substantially same time. In the multi-beam type head, the two light sources perform the recording and the reproduction for the recording validation, and thus the disk does not need to be rotated again for the recording validation. In the example of JP-A-64-70936, a high output laser beam source and an optical system having a high optical efficiency for radiating a laser beam from the light source on the disk efficiently are necessary. On the other hand, the optical efficiency of the optical system may be low regarding a laser beam performing the reproduction of the information. Accordingly, a device that accomplishes the high optical efficiency only for the leaser beam performing the information recording is sufficient.
Moreover, a requirement has been increasing in recent years that both of a CD-R (Compact Disk-Recordable) that has spread as a writable optical disk and a DVD (Digital Versatile Disc/Digital Video Disc) developed as a writable optical disk in higher density need to be recorded/reproduced with the same small optical head. Since a laser wavelength suitable for the recording/reproduction of the CD-R is approximately 780 nm while a laser wavelength suitable for the recording/reproduction of the DVD is approximately 660 nm, both of the laser beam source having the wavelength of 780 nm and the laser beam source having the wavelength of 660 nm needs to be mounted on the same optical head. For example, in JP-A-10-241189 and JP-A10-289468, small optical heads are suggested in which a semiconductor laser chip for a compact disk having the wavelength of approximately 780 nm, a semiconductor laser chip for a DVD having the wavelength of approximately 660 nm and a photodetecting element are formed in a unit. Normally, beams having different radiation points pass the different positions of the lens system. Similarly, in the optical heads, the beams radiated from the two semiconductor laser chips are made to be incident to the different positions of a focusing lens. Then, optical paths of the two beams are unified by using a transmissive hologram (grating) in JP-A-10-241189 and synthesis means by a polarizable prism (birefringent plate) or a hologram in Japanese Patent Laid-Open 10-289468. Furthermore, FIG. 6 of JP-A-10-289468 and FIG. 3 of JP-A-10-261240 show that the beam position from the laser beam source arranged outside an axis and the beam position from the laser beam source arrange on the axis are unified in the vicinity of the focusing lens by allowing the distance between the laser beam source, the collimating lens and the focusing lens to be substantially equal.